A Far Planet’s Puzzling Clouds

byPaul GilsteronFebruary 22, 2007

Our first ‘sniffs of air from an alien world,’ as David Charbonneau calls them, have brought with them a bit of a surprise. Charbonneau (Harvard-Smithsonian Center for Astrophysics) is one of a team of astronomers who have measured the spectrum from the atmosphere of a transiting exoplanet. What the team expected to find was evidence of common molecules like water, methane and carbon dioxide. Yet the scientists found none of these. The spectrum they acquired was flat.

HD 189733b is the world in question, orbiting a star about sixty light years from Earth in the constellation Vulpecula. The transiting planet is a ‘hot Jupiter,’ slightly larger and more massive than Jupiter itself, orbiting once every two days about three million miles from its star. This remarkable work consisted of studying the so-called ‘secondary eclipse’ that occurs when the planet disappears behind the star, thus extracting the planetary data from the much brighter stellar signature.

Here’s the method, as described by Charbonneau colleague Carl Grillmair (Caltech):

“Normally, trying to see a planet next to a star is like trying to see a firefly next to an airport searchlight several miles away. But in the case of our planet and the one being reported by the other teams, you can take the combined spectrum of the star and planet, and then when the planet passes behind the star, take another spectrum. By subtracting the second spectrum of just the star from the first, you can divine the spectrum of the planet itself.”

What remarkable work with the Spitzer Space Telescope, which seems more capable than anyone had imagined (and recall that transiting exoplanets had yet to be discovered when Spitzer was designed). The key to these studies is the use of infrared. At infrared wavelengths, a planet is much brighter in comparison to its star than in visible light. The method worked perhaps beyond expectation, but the result was still unusual: Looking for water vapor in the data and a prominent methane signature, the team found its results indicative of something else, something that is most likely blocking these molecules from detection.

Image: This artist’s concept shows a cloudy Jupiter-like planet that orbits very close to its fiery hot star. NASA’s Spitzer Space Telescope was recently used to capture spectra, or molecular fingerprints, of two “hot Jupiter” worlds like the one depicted here. This is the first time a spectrum has ever been obtained for an exoplanet, or a planet beyond our solar system. Credit: NASA/JPL-Caltech/T. Pyle (SSC).

And what could that something be? Helping to untangle the puzzle is the spectrum of a different planet, HD 209458b, which is under investigation both by Jeremy Richardson (GSFC) and colleagues and a JPL team led by Mark Swain. Their spectra show silicates — molecules containing silicon and oxygen — which may exist on this planet as dust grains from which clouds can form. So what’s going on in these separate spectral studies may be the marker of an interesting cloud structure. “We think that both planets may be cloaked in dark silicate clouds,” said Charbonneau. “These worlds are blacker than any planet in our solar system.”

Additional data (Grillmair and Charbonneau, for example, have only been able to observe their planet for 12 hours during two eclipses) should help clarify the situation. The Grillmair and Charbonneau paper “A Spitzer Spectrum of the Exoplanet HD 189733b” is to appear in the Astrophysical Journal Letters, and is available here as a preprint. The GSFC study is “A spectrum of an extrasolar planet”, appearing in Nature 445 (22 February 2007), pp. 892-895, abstract here. A separate paper on HD 209458b by Mark Swain’s team at JPL is to appear in Astrophysical Journal Letters (no preprint yet available).

Comments on this entry are closed.

ljkFebruary 23, 2007, 10:14

Astrophysics, abstract
astro-ph/0702593

From: Rachel Akeson [view email]

Date: Wed, 21 Feb 2007 22:19:37 GMT (209kb)

The mid-infrared spectrum of the transiting exoplanet HD 209458b

Authors: M.R. Swain, J. Bouwman, R. Akeson, S. Lawler, C. Beichman

Comments: 13 pages, submitted to the Astrophysical Journal Letters

We report the spectroscopic detection of mid-infrared emission from the transiting exoplanet HD 209458b. Using archive data taken with the Spitzer/IRS instrument, we have determined the spectrum of HD 209458b between 8.25 and 13.25 micron with an average SNR of ~4 in each 0.25 micron spectral channel. We have used two independent methods to determine the planet spectrum and find the results are in good agreement. In the mid-infrared, the planet spectrum is dominated by thermal emission with a temperature consistent with previous estimates.

The absence of strong spectral features is significant and is most consistent with emission at these wavelengths originating primarily from optically thick clouds located at relatively high elevation in the planet’s atmosphere. This work required development of improved methods for Spitzer/IRS data calibration that increase the achievable dynamic range for observations of bright point sources.

We use a grid-based shallow water model to simulate the atmospheric dynamics of the transiting hot Jupiter HD 209458b. Under the usual assumption that the planet is in synchronous rotation with zero obliquity, a steady state is reached with a well-localized cold spot centered 76 degrees east of the antistellar point. This represents a departure from predictions made by previous simulations in the literature that used the shallow water formalism; we find that the disagreement is explained by the factor of 30 shorter radiative timescale used in our model. We also examine the case that the planet is in Cassini state 2, in which the expected obliquity is ~90 degrees. Under these circumstances, a periodic equilibrium is reached, with the temperature slightly leading the solar forcing. Using these temperature distributions, we calculate disk-integrated bolometric infrared light curves from the planet. The light curves for the two models are surprisingly similar, despite large differences in temperature patterns in the two cases. In the zero-obliquity case, the intensity at the minimum is 66% of the maximum intensity, with the minimum occuring 72 degrees ahead of transit. In the high-obliquity case, the minimum occurs 54 degrees ahead of transit, with an intensity of 58% of the maximum.

Flagstaff, Ariz. â€“ For the first time, water has been identified in the
atmosphere of an extrasolar planet. Through a combination of
previously published Hubble Space Telescope measurements and new
theoretical models, Lowell Observatory astronomer Travis Barman has
found strong evidence for water absorption in the atmosphere of
transiting planet HD209458b. This result was recently accepted for
publication in the Astrophysical Journal

“We now know that water vapor exists in the atmosphere of one
extrasolar planet and there is good reason to believe that other
extrasolar planets contain water vapor,” said Barman.

Water vapor (or steam) has been expected to be present in the
atmospheres of nearly all of the known extrasolar planets, even those
that orbit closer to their parent star than Mercury is to our Sun. For
the majority of extrasolar planets, their close proximity to their
parent star has made detecting water and other compounds difficult.
The identification reported here takes advantage of the fact that
HD209458b, as seen from Earth, passes directly in front of its star
every three and half days. As a planet passes in front of a star, its
atmosphere blocks a different amount of the starlight at different
wavelengths. In particular, absorption by water in the atmosphere of a
giant planet makes the planet appear larger across a specific part of
the infrared spectrum compared to wavelengths in the visible spectrum.
An analysis of visible and infrared Hubble data carried out last year
by Harvard student Heather Knutson made possible a direct comparison to
new theoretical models developed by Barman at Lowell Observatory. This
ultimately led to the identification of water absorption in a planet
150 light years from Earth.

â€œIt is encouraging that theoretical predictions of water in extrasolar
planets seem to agree reasonably well with observations,â€ said Barman.

This research was supported by NASAâ€™s Origins of Solar System program.

Abstract: Precision infrared photometry from Spitzer has enabled the first direct studies of light from extrasolar planets, via observations at secondary eclipse in transiting systems. Current Spitzer results include the first longitudinal temperature map of an extrasolar planet, and the first spectra of their atmospheres. Spitzer has also measured a temperature and precise radius for the first transiting Neptune-sized exoplanet, and is beginning to make precise transit timing measurements to infer the existence of unseen low mass planets. The lack of stellar limb darkening in the infrared facilitates precise radius and transit timing measurements of transiting planets. Warm Spitzer will be capable of a precise radius measurement for Earth-sized planets transiting nearby M-dwarfs, thereby constraining their bulk composition. It will continue to measure thermal emission at secondary eclipse for transiting hot Jupiters, and be able to distinguish between planets having broad band emission versus absorption spectra. It will also be able to measure the orbital phase variation of thermal emission for close-in planets, even non-transiting planets, and these measurements will be of special interest for planets in eccentric orbits. Warm Spitzer will be a significant complement to Kepler, particularly as regards transit timing in the Kepler field. In addition to studying close-in planets, Warm Spitzer will have significant application in sensitive imaging searches for young planets at relatively large angular separations from their parent stars.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to your fellow readers, I'm probably not going to run it.